The invention concerns radial tires whose crown is reinforced under the tread by means of superimposed plies of cords or cables which, in what follows, constitute the crown reinforcement. It also concerns a method for mounting a train of tires on a road vehicle.
As a general rule a tire for a passenger car comprises a radial carcass reinforcement anchored in two tire beads and covered radially by a crown reinforcement, itself covered radially by a tread designed to come into contact with the road during rolling. The crown reinforcement comprises at least two plies, known as working plies, each ply being composed of a plurality of cords or cables essentially parallel to one another within one ply, the cords or cables of one ply being crossed over relative to the cords or cables of the adjacent ply or plies.
To improve the general rolling characteristics of a vehicle fitted with tires, it is known to use tires having structural asymmetries.
To solve a wear problem on a vehicle rolling on a path with relatively small radii of curvature, FR 1.342.822 describes the formation of an asymmetrical crown reinforcement in which that the transverse rigidity of the tread decreases from one edge to the other with the less rigid portion facing outwards on the vehicle to which the said tire is fitted. To obtain this asymmetry, it is proposed among the examples described, to form a crown reinforcement with one ply extending across the full width of the tread and a second ply across only part of the said width, so as to confer on the said tread a greater transverse rigidity on one side than on the other.
Although interesting in terms of rolling behavior performance on the vehicle, the presence of asymmetrical rigidity in a tire has certain disadvantages and in particular that of drift when the said vehicle is rolling in a straight line (that is to say, a stress distribution in the tread essentially equivalent to that produced when going around a curve). This leads to contact forces which are detrimental from the standpoint of tire tread wear; the forces exerted by the road on the vehicle""s tires also cause the vehicle to veer (that is, it deviates progressively from its path during rolling in a straight line).
To address this problem, FR 1.444.271 proposes a means to attenuate the effects of an asymmetrical structure when rolling in a straight line, by superimposing over the said structure a second asymmetry in order to counterbalance the effects of the first asymmetry while preserving good rolling behavior performance around bends for example, due to the presence of the first asymmetry. The examples described involve either the addition of a supplementary reinforcement element (carcass ply extending beneath the crown reinforcement), or additional thickness of the tread located over only part of the said tread.
The purpose of the present invention is to propose an asymmetrical tire structure that performs well both when rolling on bends and when travelling in a straight line, the said structure considerably reducing the lateral pull of a vehicle fitted with this type of tire and therefore avoiding the appearance of irregular wear linked for example to drift while rolling in a straight line. It has been found that by choosing particular values for a certain number of parameters, a considerable performance improvement can be obtained while preserving some asymmetry of the tire""s structure without making the said tire any heavier.
This objective is achieved with a tire that has a preferred direction of fitting to a road vehicle. The tire comprises two tire beads, a crown and two side-walls that connect the beads and the crown, the said tire having a carcass reinforcement that extends from one bead to the other and, radially outside it, a crown reinforcement of width Ls formed by a stack of at least two crown plies, each of these crown plies having a width equal to or greater than Ls and comprising a plurality of cords or cables arranged parallel to one another along essentially the same direction, the said cables crossing over from one ply to the next.
This tire also comprises a meridian reinforcing ply arranged radially between the carcass reinforcement and the crown reinforcement, and comprising a plurality of cords or cables essentially parallel to one another and making with respect to the circumferential direction an average angle between 50xc2x0 and 90xc2x0, the said ply being of width Lr measured between a first edge and a second edge, both edges being located on the same side relative to the equatorial plane of the tire.
The tire is characterized in that it comprises marking to indicate the side of the tire on which the meridian reinforcing ply is located (e.g., xe2x80x9cinner sidexe2x80x9d), this marking enabling the said ply to be positioned on the inside of the vehicle when the tire is mounted on the said road vehicle.
The width Ls of the crown reinforcement is taken as the smallest of the widths of the crown plies, and corresponds to the effective width of the tire, that is, the width over which there is mechanical coupling between the crown plies.
While the state of the known prior art discloses tires with crowns comprising plies called triangulation plies and arranged symmetrically relative to the equatorial plane of the said tires, the present invention proposes a novel and not obvious solution which consists in associating the presence of a single meridian reinforcing ply with an indication of the necessary position of this ply relative to the vehicle, once the tire has been fitted to the vehicle, that will lead to good rolling performances both in a straight line and when rounding bends, without giving rise to undesirable wear and lateral pull effects.
Various marking means can be used, in particular marking on one side-wall of the tire to indicate the side on which the meridian reinforcing ply is located, a specific marking on the outside or inside surface of the tire indicating the side of the tire that must be positioned outwards or inwards on the vehicle, granted that the meridian reinforcing ply has to be on the inside.
With the tire according to the invention, the following characteristics can also be applied, alone or in combination:
the width Lr of the meridian reinforcing ply is between 15% and 45% of Ls;
the first edge of this meridian reinforcing ply is located a distance D1 from the equatorial plane between 35% and 50% of Ls, while the second edge of the meridian reinforcing ply is located between the first edge and the equatorial plane of the tire;
the mean compression rigidity of the meridian reinforcing ply in the direction of the cords or cables of the said ply is higher than 1 Gpa, this rigidity being obtained by multiplying the compression rigidity of a single cord or cable by the number of cords or cables per unit length of the ply.
The combination of the parameters mentioned above makes it possible to benefit from the advantages of an asymmetrical tire structure without the need for additional means to counterbalance the effects of the drift induced during rolling in a straight line. The advantage of a tire according to the invention is significant, since the phenomena of irregular wear while rolling in a straight line (in the sense of wear localized in certain areas of the tread) are appreciably reduced.
If Lr is less than 15% of Ls, the reinforcing effect of the additional meridian reinforcing ply is insufficient for its effect on the tire to be measured; if Lr is more than 45% of Ls, when the tire is rotating at high speed effects are induced which can affect the tire""s rolling performance adversely.
Similarly, to avoid any blockage in the radial direction of the median portion of the tire""s crown when it is rotating about its axis, it is preferable to provide that the second edge of the meridian reinforcing ply is located at a distance D2 from the equatorial plane larger than one-third of Lr.
When a tire according to the invention is rotating at a high speed, the presence of a meridian reinforcing ply has an effect by virtue of its additional mass alone compared with the same tire without the said ply. To reduce this effect as much as possible, it is preferable to provide that the edge of the meridian reinforcing ply furthest away from the equatorial plane is located a distance D1 from the said plane between 35% and 45% of the total width Ls of the crown reinforcement.
To lighten the crown reinforcement and hence also the tire substantially, it is advantageous to provide that the crown reinforcement embodies at least two crown plies, each crown ply being composed of a rubber matrix reinforced by a plurality of textile cords or cables arranged parallel to one another along essentially the same direction, the meridian reinforcing ply being reinforced by metallic wires or cables. This arrangement makes it possible to confer upon the crown reinforcement a satisfactory level of flexural rigidity on edge, also making it possible to reduce considerably the rolling resistance of a tire comprising the said crown reinforcement. The same positive results are achieved if rubber mixes having low modulus of elasticity are used in combination, the said mixes covering the textile cables of the working plies.
Preferably, the rubber mix of the crown reinforcement plies and of the meridian reinforcing ply has a tensile modulus at 10% deformation and ambient temperature below 12 Mpa, and a loss at 60xc2x0 C. below 25%.
Advantageously, the width Lr of the additional reinforcing ply with essentially meridian orientation is between 20% and 35% of the effective width of the crown reinforcement comprising the plies known as working plies.
An optimum effect is achieved when the mean angle of the reinforcements (in the form of cords or cables) of the additional meridian reinforcing ply is between 70xc2x0 and 90xc2x0, the said angle being measured relative to the equatorial plane perpendicular to the rotation axis of the tire and passing through the middle of the width of the crown reinforcement.
In addition, radially between the crown reinforcing ply and the carcass reinforcement there may be arranged a rubber section with a thickness between 0.3 and 1 mm (this thickness being measured radially above the axially outermost edge of the reinforcing ply), having a tensile modulus at 10% deformation, measured under normal test conditions and at ambient temperature, at least equal to 5 Mpa and at most equal to 15 Mpa, the said section extending continuously in the circumferential direction and, when viewed in a radial section plane (i.e. one containing the rotation axis of the tire), having its axially innermost edge located at a distance from the equatorial plane smaller than the distance of the axially outermost edge of the reinforcing ply measured relative to the same equatorial plane.
It is also interesting to associate with the tire according to the invention the presence of a tread pattern, that part, for example half of the said tread situated radially on the outside of the additional, meridian reinforcing ply being provided with a pattern of its own formed of relief elements, the said pattern having a tread rigidity less than that of the other half of the tread; in this case, the thicknesses of the tread are essentially constant across the full width of the said tread. xe2x80x9cTread rigidityxe2x80x9d means essentially the rigidity of the tread when the tire according to the invention is subjected to drift forces under normal conditions of use (inflation pressure and supported load).
In addition, a method is proposed for mounting a group of four tires with asymmetrical structure, that gives improved rolling performance around bends while reducing the tire wear and the lateral pull of the vehicle, each tire having two beads, a crown and two side-walls that connect the beads to the crown and each tire having a carcass reinforcement which extends from one bead to the other and, radially outside this, a crown reinforcement of width equal to Ls, formed by a stack of at least two crown plies, each of the said crown plies having a width greater than or equal to Ls and being composed of a plurality of cords or cables arranged parallel to one another along essentially the same direction, with the cables crossed over from one ply to the next.
Each tire also comprises a meridian reinforcing ply positioned radially between the carcass reinforcement and the crown reinforcement and formed of a plurality of cords or cables essentially parallel to one another and forming an average angle with respect to the circumferential direction between 50xc2x0 and 90xc2x0, this ply of width Lr being situated entirely on one side of the equatorial plane of the tire.
The method according to the invention involves mounting four tires as described, in such manner that the meridian reinforcing ply of each tire mounted on the vehicle is located on the inside of the vehicle, i.e. between the inside of the vehicle and the equatorial plane of the tire.
By definition, the equatorial plane is the plane perpendicular to the tire""s rotation axis that passes through the middle of the crown reinforcement.
Preferably, the width Lr, measured between a first edge and a second edge, is between 15% and 45% of Ls, the first edge of this meridian reinforcing ply being located a distance D1 from the equatorial plane of the tire between 35% and 50% of Ls, and the second edge of the meridian reinforcing ply being situated between the first edge and the equatorial plane of the tire.
Preferably, for each tire the mean compression rigidity of the meridian reinforcing ply in the direction of its cords or cables is greater than 1 Gpa, this rigidity being obtained by multiplying the compression rigidity of a single cord or cable by the number of cords or cables per unit length of the ply.
The method proposed, according to which the meridian reinforcing plies of the four tires of a vehicle are positioned towards the inside of the vehicle, also applies to the fitting of the tires of the front axle of the said vehicle, although it does not result in the same performance level for the vehicle.